Background. Frequent premature ventricular contractions (PVCs) can cause LV dysfunction (CM), referred to as PVC-induced cardiomyopathy (CM). The mechanism responsible and the impact of PVC coupling interval (prematurity) are unclear. Suspected triggers are post-extrasystolic potentiation and LV dyssynchrony. Hypotheses. Our working hypotheses are: 1) Post-extrasystolic potentiation, associated with PVCs, is a key trigger responsible for PVC-induced CM. LV dysfunction (Aim 1), impaired EC coupling and dyad remodeling (Aim 2) are greater in short- rather than long- coupled PVCs since this phenomenon is more prominent in short- rather than long-coupled PVCs. 2) Impaired EC coupling in PVC-induced CM is due to changes in JPH-2 and Cav1.2 pathways (Fig.1). 3) Early changes in dyad, JPH-2 and Cav1.2 pathways precede the development of this CM and the recovery of LV function after PVC cessation, providing evidence that these changes are a primary cause of CM. 4) Decrease in post-extrasystolic potentiation (less Ca2+ overload) with diltiazem (LTCC blocker) results in a lesser dyad remodeling, thereby preventing or minimizing impaired EC-coupling and PVC-induced CM. Aim 1. Evaluate the impact of post-extrasystolic potentiation and PVC coupling interval in the development of LV dysfunction associated with chronic exposure to frequent PVCs (PVC-induced CM). Aim 2. Determine the temporal structural and molecular changes responsible for decrease in Ca2+ release and Junctophylin-2, and their role on the pathophysiology of PVC-induced CM and recovery upon PVC cessation. Aim 3. Assess the impact of reducing post-extrasystolic potentiation with L-type Ca2+ channel blocker to prevent abnormal Ca2+ release and EC-coupling in PVC-induced CM. Methods. Fifty four canines will undergo pacemaker implant to reproduce 50% PVCs (ventricular bigeminy). They will be randomized to one of 4 groups: 1) short-coupled PVC (n=14), 2) long-coupled PVCs (n=14), 3) short-coupled PVCs + diltiazem (n=14), or 4) control (n=12) groups. At the end of a 12-week PVC period, a recovery phase (disabling PVCs) will be allowed in 50% of animals. Serial cardiac evaluation and biopsies will allow us to assess LV function, dyad structure, Ca2+ transients (EC coupling), changes in JPH-2 and Cav1.2 expression, function and distribution and their mediators at different time points of CM in all groups. Significance. This study will: 1) provide an understanding of the role of post-extrasystolic potentiation and LV dyssynchrony in the mechanism of PVC-induced CM; 2) assess the impact of PVC coupling interval in the development or severity of PVC-induced CM, 3) identify molecular mechanisms behind impaired EC coupling in PVC-induced CM and 4) assess the benefit of LTCC blockers to prevent or minimize PVC-induced CM. Understanding the mechanism of PVC-induced CM will help us to identify high-risk patients for development of PVC-induced CM, but most importantly find future targets to prevent and treat subjects with PVC-induced CM.